sustainability

Article

A Comparison of Various Technological Options forImproving Energy and Water Use Efficiency in aTraditional Sugar Mill

Eyerusalem Birru 1,*, Catharina Erlich 1, Idalberto Herrera 2, Andrew Martin 1, Sofia Feychting 3,Marina Vitez 3, Emma Bednarcik Abdulhadi 3, Anna Larsson 3, Emanuel Onoszko 3,Mattias Hallersbo 3, Louise Weilenmann 3 and Laura Puskoriute 3

1 Department of Energy Technology, KTH Royal Institute of Technology,School of Industrial Technology and Management (ITM), 10044 Stockholm, Sweden;erlich.catharina@energy.kth.se (C.E.); andrew@energy.kth.se (A.M.)

2 Center for Energy and Environmental Technologies Assessment (CEETA),Faculty of Mechanical and Industrial Engineering Central, University “Marta Abreu” of Las Villas (UCLV),54830 Santa Clara, Cuba; iherrera@kth.se

3 KTH Royal Institute of Technology, School of Industrial Technology and Management (ITM),10044 Stockholm, Sweden; sofiafe@kth.se (S.F.); vitez@kth.se (M.V.); emmabed@kth.se (E.B.A.);annala8@kth.se (A.L.); eonoszko@kth.se (E.O.); hallersb@kth.se (M.H.); louiseny@kth.se (L.W.);laurapu@kth.se (L.P.)

* Correspondence: birru@kth.se or eyerusalem.deresse.birru@energy.kth.se; Tel.: +46-8-790-7428

Academic Editor: Alessandro FrancoReceived: 31 August 2016; Accepted: 4 November 2016; Published: 25 November 2016

Abstract: This study is a comparison of four technological improvements proposed in previousworks for the Cuban sugar mill Carlos Baliño. These technological options are: (1) utilization ofexcess wastewater for enhanced imbibition; (2) utilization of waste heat for thermally driven cooling;(3) utilization of excess bagasse for pellets; and (4) modification of the cogeneration unit for maximumelectric power generation. The method used for the evaluation of the technological options involvesusing criteria such as energy saving, financial gains, and CO2 emission saving potential. The resultsof the analysis show that the first three technological improvement options are attractive only duringthe crushing season. On the other hand, the last technological improvement option can be attractiveif a year round generation of surplus power is sought. The first technological improvement optionleads to only minor changes in energy utilization, but the increase in sugar yield of 8.7% leads toattractive profitability with an extremely low payback period. The CO2 emissions saved due to thefourth technological improvement option are the highest (22,000 tonnes/year) and the cost of CO2

emissions saved for the third technological improvement option (lowest) amount to 41 USD/tonneof CO2 emissions saved. The cycle efficiencies of the third and fourth technological improvementoptions are 37.9% and 36.8%, respectively, with payback periods of 2.3 and 1.6 years. The secondtechnological improvement option is the least attractive alternative of the group.

Keywords: sugar cane bagasse; Carlos Baliño; energy efficiency; wastewater reuse; imbibition;CO2 emission; absorption chiller; pellet; electricity; energy saving; payback period

1. Introduction

Global economic expansion and population growth have resulted in an increased demand of twovaluable resources: energy and water. These two resources are interdependent in that water is a vitalinput for production of energy services, and energy is crucial for the provision of water [1]. Concernabout the limited supply of these resources and climate change has led to measures such as energy

Sustainability 2016, 8, 1227; doi:10.3390/su8121227 www.mdpi.com/journal/sustainability

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efficiency improvements, use of renewable energy based technologies and water use optimization.Currently, such measures are being widely investigated and implemented in various industriesworldwide. The sugar cane industry is one example where energy and water are both intensivelyused, and also wasted. Efficient utilization of these resources is crucial for maximizing the energyefficiency of sugar mills and for reducing emissions. Several studies have considered energy efficiencyimprovement measures by mainly focusing on maximization of electric power generation [2–5]. On theother hand, efficient utilization of water in sugar cane mills is not widely addressed compared toenergy efficiency.

Efficient utilization of energy and water in the sugar cane industry is a key measure for tacklingthe problem of limited access to these resources in developing countries, where such industries arewidely found. Cuba is one of these developing countries with sugar cane industries and strugglingwith limited access to renewable resource-based energy and water. The main source of energy inthe Cuban energy supply is based on fossil fuels, and since 2005 there have been several energyefficiency improvement projects initiated by the government, which resulted in a decreased numberof blackouts [6]. The sugar cane industry in Cuba can be one potential sector to implement energyefficiency measures with the aim of contributing to green electricity supply, reusing wastewater andutilizing the bagasse potential. The sugar mill Carlos Baliño has recently been upgraded, with electricdrives replacing the old mechanical turbines and a new boiler. However, there was no upgrading ofthe turbines with subsequent efficiency improvement, so plenty of excess bagasse is currently beingproduced. Part of this bagasse is used as maintenance fuel as well as a sellable fuel for nearby sugarmills, yet a significant portion piles up as waste. Moreover, this mill retains an outdated approach towater treatment, as excess process water is simply led to ponds for evaporation.

In an attempt to address the above mentioned issues with the Carlos Baliño sugar mill, previouswork had been done [7–10] where specific technological improvements were analyzed. Thesetechnological improvements, which were considered as part of field studies conducted during 2014 and2015 [7–10], include the following: utilization of excess wastewater for enhanced imbibition; utilizationof waste heat for thermally driven cooling; bagasse pelletization; and bagasse drying for surpluspower generation. Parameters for analysis and comparison include energy performance, economicconsiderations, and CO2 emissions. The main objective of this study is to make a comparison of thesetechnological improvements in water and energy utilization for the Carlos Baliño sugar mill, with theultimate goal of assessing the most viable technologies among the alternatives.

2. Background and Methods

The case study plant, Carlos Baliño sugar mill, is located in Villa Clara, Cuba. It was built in 1903and in 2001 it started producing ECOCERT-labeled organic sugar [11], which now accounts for 30% ofits total sugar production. The sugar mill is known for its high efficiency equipment and high qualityproduct. The cane crushing capacity is 2300 TCD (Tonne of cane per day) and the crushing season isbetween mid-December to April. The process steam pressure is 2.05 bar (abs.).

Figure 1 shows the process flow sheet for Carlos Baliño sugar mill. The process flow sheetindicates also the wastewater sinks and sources of the sugar mill (see [7,8] for more information).The sugar production process is described in the following sub-sections.

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Figure 1. Flow sheet of Carlos Baliño sugar mill [8].

2.1. Sugar Production Processing Steps at Carlos Baliño Sugar Mill

In this section, the sugar production processes with some operation parameters are provided. All the information provided in this section is obtained (unless specified) from the documents collected during field visit to Carlos Baliño sugar mill.

2.1.1. Cane Preparation

The cane harvest is done beginning mid of December and lasts through out April each year [7]. The cane harvest uses mechanized harvest method. The transportation of the harvested cane to the cane cleaning unit is carried out using trucks and carts. The cane cultivation is done using organic agriculture methods. In the cleaning center, more than 50% of the foreign matter such as straw and soil is removed.

Clean cane is transported by trucks and unloaded to the cane feed conveyer where it is split and fed to the cane levelizer and subsequently rendered as cane sheets before reaching the cane knives. The cane knives are powered by two electric motors, 250 and 320 kW at 600 rpm and 440 V. The chopped cane is sent to the cane mill, comprised of a cane crusher and three roller mills driven by two electric motors (400 kW, 440 V and 600 rpm). Imbibition water at about 70 °C and a ratio of twice the fiber is added at the inlet of the third mill. Extracted juice from the second and third roller mills is recirculated to the adjacent mill. At this stage of the extraction process, besides sugar juice, bagasse is obtained and a part of the latter is sent to the boiler to generate electricity in addition to process steam. The other part of bagasse is intended for the production of compost and surplus for sale to other companies, with the remainder eventually landfilled.

2.1.2. Juice Purification

Next, the juice is heated in juice heaters to 105 °C to promote reaction between milk of lime and phosphorous present in the juice. The juice then enters the clarifier where alkalizing and heating is done. The heated clear juice enters two rotary filters and two streams are produced: sludge/filter cake and the clarified filtered juice. This juice with 15 °Bx is sent to the evaporators for juice concentration.

Figure 1. Flow sheet of Carlos Baliño sugar mill [8].

2.1. Sugar Production Processing Steps at Carlos Baliño Sugar Mill

In this section, the sugar production processes with some operation parameters are provided.All the information provided in this section is obtained (unless specified) from the documents collectedduring field visit to Carlos Baliño sugar mill.

2.1.1. Cane Preparation

The cane harvest is done beginning mid of December and lasts through out April each year [7].The cane harvest uses mechanized harvest method. The transportation of the harvested cane to thecane cleaning unit is carried out using trucks and carts. The cane cultivation is done using organicagriculture methods. In the cleaning center, more than 50% of the foreign matter such as straw and soilis removed.

Clean cane is transported by trucks and unloaded to the cane feed conveyer where it is splitand fed to the cane levelizer and subsequently rendered as cane sheets before reaching the caneknives. The cane knives are powered by two electric motors, 250 and 320 kW at 600 rpm and 440 V.The chopped cane is sent to the cane mill, comprised of a cane crusher and three roller mills drivenby two electric motors (400 kW, 440 V and 600 rpm). Imbibition water at about 70 ◦C and a ratio oftwice the fiber is added at the inlet of the third mill. Extracted juice from the second and third rollermills is recirculated to the adjacent mill. At this stage of the extraction process, besides sugar juice,bagasse is obtained and a part of the latter is sent to the boiler to generate electricity in addition toprocess steam. The other part of bagasse is intended for the production of compost and surplus forsale to other companies, with the remainder eventually landfilled.

2.1.2. Juice Purification

Next, the juice is heated in juice heaters to 105 ◦C to promote reaction between milk of lime andphosphorous present in the juice. The juice then enters the clarifier where alkalizing and heating isdone. The heated clear juice enters two rotary filters and two streams are produced: sludge/filter cakeand the clarified filtered juice. This juice with 15 ◦Bx is sent to the evaporators for juice concentration.

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2.1.3. Juice Concentration

The clear juice is fed to two pre-evaporators after heating it to 106–108 ◦C. These pre-evaporatorsoperate in parallel receive exhaust steam with 15 Psi and produce juice with 8 Psi. The syrup leavesthe evaporators with 60–63 ◦Bx.

The syrup is then sent to the crystallization unit and centrifugation. Sugar coming out of thecentrifuges usually has a moisture content of 0.5%–2%. The sugar is dried using hot air to 42 ◦C anda reduced moisture content of 0.2%–0.5% may be achieved. Drying is usually carried out in a directcontact rotary drum dryer. This reduced moisture content of sugar is critical to prevent the growth ofmicroorganisms and hardening that could cause damage in the stored product and improves handling.The separation of ferrous particles, which might come from cane handling and conveying equipment,is done by a self-cleaning magnetic separator located at the dryer outlet, resulting in 0 ppm Fe in thepackaged sugar. The sugar is then sieved and packed.

2.2. Cogeneration at Carlos Baliño Sugar Mill

Steam is generated by combusting bagasse in suspension-type boiler with a temperature of340–360 ◦C and a pressure of 18 bar. The boiler brand is RETAL, manufactured in Germany at theend of the eighties, converted into a CAP60 in 2008 in the steam boiler factory of Sagua la Grande,Cuba, originally designed to generate 45 tonnes/h. The boiler was converted to generate 60 ton/hsteam at the same conditions of pressure and temperature. There are two back pressure steam turbines,manufactured in Germany in 1884, VEB Bergmann-Borsig brand, SG 49/3/6 model. These have aninstalled nominal capacity of 3.16 MW and specific electricity provision of 32 kWh/TC. Excess power issent to the national grid [12]. The existing back pressure power turbines have a limitation in generatingexcess power more than the current amount [10]. The feed water pumps are KSB brand, series HGMand are manufactured in Germany.

The cogeneration unit of traditional sugar mills is commonly equipped with back pressureturbines for power generation, and the main focus is the production of electricity for the factory’sconsumption. Surplus power generation does not occur often, and only during the crushing season.The use of condensing extraction steam turbines however allows sugar mills to generate year roundpower generation [4]. During unplanned stoppages the cogeneration unit does not normally stopfully, rather the bagasse flow to the boiler will decrease, thus the boiler runs at a lower load. There areconditions where sugar mills use fuel oil during unplanned stoppages, incurring an extra cost andresulting in relatively high carbon dioxide emissions [13]. This is not the case in Carlos Baliño as theexisting boilers can use only bagasse or wood. Thus, stored bagasse or purchased wood is used duringunplanned stoppages as well as during boiler startups. On average, company records show that thefactory operates at 70% of target capacity when including complete shutdowns in the statistics [7,8].

2.3. Technological Improvements under Consideration

Four technological improvements as described in previous works [7–10] are discussed below.

2.3.1. Option 1: Utilization of Excess Wastewater for Enhanced Imbibition

Figure 2 illustrates the overall concept of Option 1, where the excess wastewater is recycledfor imbibition (resulting in higher sugar production). The idea for re-using the excess waste waterfor imbibition as analyzed in the previous work [7] was proposed after it was determined that theexisting imbibition water amount is not optimized and additional water is needed. As can be seenfrom Figure 1, a portion of contaminated condensate is mixed with the imbibition water supply,illustrating that excess wastewater has the potential to be recycled for imbibition without treatment.The imbibition water flow is such that there will be a maximum profit regarding the increased sugarproduction. It should be noted that there is an optimum amount of imbibition water needed formaximum sugar production. On the other hand, most sugar mills use imbibition water in excess which

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results only excess wastewater instead of increased sugar production. This is because the amount ofsucrose extracted is not only a function of amount of imbibition water but also other factors such ashydraulic pressure of the roller mills, fiber content of cane, and the amount of cane mille. Thus, beyonda certain amount of imbibition water amount, there will not be sucrose extraction. The imbibitionwater optimization was done by varying the efficiency of the milling pair, which is the result of variedmass flow of imbibition water. This in turn results in variation in the amount of produced sugar.The optimum amount of imbibition water is thus the amount that results in maximum economicgain regarding sales of sugar. After identifying the optimum amount of imbibition water, which wasestimated to be 11 kg/s (96% increase from the currently used amount), part of the excess wastewateris reused and the remaining is sent to a wastewater treatment facility.

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amount of sucrose extracted is not only a function of amount of imbibition water but also other factors such as hydraulic pressure of the roller mills, fiber content of cane, and the amount of cane mille. Thus, beyond a certain amount of imbibition water amount, there will not be sucrose extraction. The imbibition water optimization was done by varying the efficiency of the milling pair, which is the result of varied mass flow of imbibition water. This in turn results in variation in the amount of produced sugar. The optimum amount of imbibition water is thus the amount that results in maximum economic gain regarding sales of sugar. After identifying the optimum amount of imbibition water, which was estimated to be 11 kg/s (96% increase from the currently used amount), part of the excess wastewater is reused and the remaining is sent to a wastewater treatment facility.

Figure 2. Wastewater for enhanced imbibition.

2.3.2. Option 2: Utilization of Waste Heat for Thermally Driven Cooling

The aim of the study was to analyze the possibilities of utilizing the waste heat in the excess wastewater flow as a heat source for a thermally driven absorption chiller in order to provide chilled water for air conditioning to laboratories and office areas of the mill. These facilities are currently air conditioned using electrically driven vapor compression air conditioning units. In this way the internal electricity use can be reduced, since heat-driven cooling consumes much less electricity than conventional compression coolers. The thermally driven absorption chiller considered in this study is driven on low-grade heat such as the waste water heat from the sugar mill. The temperature of the heat source can be as low as 75 °C and a cooling of 6 °C is produced [8]. A cooling effect is created when heat is absorbed during the evaporation of a refrigerant at low pressure in the absorption cooling cycle. The refrigerant vapor produced is then absorbed by an absorbent and the liquid solution formed is pumped to the generator in the cycle where heat is supplied and where the refrigerant is separated from the absorbent due to evaporation at high pressure. The refrigerant vapor then goes to a condenser where heat is rejected and the absorbent continues to the absorber and the cycle repeats.

Figure 3 illustrates the overall concept of this option. In order to determine the amount of waste heat available, wastewater flows of the sugar mill were traced and calculated from available data. Wastewater flows from boiler blow down, filter mud wash and floor washing were not considered in the calculation. The waste water flows considered in the previous work [8] are from juice heaters, evaporators and crystallizers. The temperature of the wastewater which is 96 °C is amenable for absorption cooling. The procedure for the calculation of the total amount of excess wastewater flow is similar to the case of Option 1.

Figure 2. Wastewater for enhanced imbibition.

2.3.2. Option 2: Utilization of Waste Heat for Thermally Driven Cooling

The aim of the study was to analyze the possibilities of utilizing the waste heat in the excesswastewater flow as a heat source for a thermally driven absorption chiller in order to provide chilledwater for air conditioning to laboratories and office areas of the mill. These facilities are currentlyair conditioned using electrically driven vapor compression air conditioning units. In this way theinternal electricity use can be reduced, since heat-driven cooling consumes much less electricity thanconventional compression coolers. The thermally driven absorption chiller considered in this study isdriven on low-grade heat such as the waste water heat from the sugar mill. The temperature of theheat source can be as low as 75 ◦C and a cooling of 6 ◦C is produced [8]. A cooling effect is createdwhen heat is absorbed during the evaporation of a refrigerant at low pressure in the absorption coolingcycle. The refrigerant vapor produced is then absorbed by an absorbent and the liquid solution formedis pumped to the generator in the cycle where heat is supplied and where the refrigerant is separatedfrom the absorbent due to evaporation at high pressure. The refrigerant vapor then goes to a condenserwhere heat is rejected and the absorbent continues to the absorber and the cycle repeats.

Figure 3 illustrates the overall concept of this option. In order to determine the amount of wasteheat available, wastewater flows of the sugar mill were traced and calculated from available data.Wastewater flows from boiler blow down, filter mud wash and floor washing were not consideredin the calculation. The waste water flows considered in the previous work [8] are from juice heaters,evaporators and crystallizers. The temperature of the wastewater which is 96 ◦C is amenable forabsorption cooling. The procedure for the calculation of the total amount of excess wastewater flow issimilar to the case of Option 1.

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Sugar millSugar cane Waste HeatAbsorption

chillerChilled water

Excess bagasse

Surplus electricity

Ele

ctri

c po

wer

Figure 3. Sugar mill waste heat for thermally driven cooling.

2.3.3. Option 3: Utilization of Excess Bagasse for Pellet

Similar to any biomass, bagasse has a bulky nature which makes its handling and transportation a bit difficult. A more compact form of the bagasse in the form of pellets have the advantage of improving its handling and transportation [14]. In addition, bagasse pellets can be stored for utilization during the off-season. The fact that bagasse pellets have a much lower moisture content than the raw bagasse improves combustion in the boiler. The bagasse pelletization process basically involve drying, milling, conditioning and cooling. The heat needed for drying before pelletization of the bagasse is assumed to be obtained from flue gas heat recovery and the optimum particle size is recommended to be less than 3 mm [15]. More information on the pelletization principle can be found on [9,15]. The idea here is to pelletize excess bagasse for sale to various customers. Figure 4 illustrates the concept behind this option.

Figure 4. Utilization of excess bagasse for pellets.

2.3.4. Option 4: Modification of the Cogeneration Unit for Maximum Electric Power Generation

The aim of this option is to maximize electric power generation via dried bagasse and installation of a condensing extraction steam turbine (CEST). The bagasse drying results in reduced moisture content (MC) of the bagasse which in turn leads to improved heating value. This improves the combustion temperature and boiler efficiency. As the final exhaust steam pressure from CEST is below atmospheric, the expansion of live steam down to this pressure will generate more power than if the same amount of live steam with the same conditions were expanded in a back-pressure turbine. In addition, in the case where there is excess bagasse available for use during off-season, the CEST will enable year round generation of electricity for sales to the grid regardless of the absence of sugar/ethanol production. Figure 5 illustrates the concept behind the technological improvement 4.

Figure 3. Sugar mill waste heat for thermally driven cooling.

2.3.3. Option 3: Utilization of Excess Bagasse for Pellet

Similar to any biomass, bagasse has a bulky nature which makes its handling and transportation abit difficult. A more compact form of the bagasse in the form of pellets have the advantage of improvingits handling and transportation [14]. In addition, bagasse pellets can be stored for utilization duringthe off-season. The fact that bagasse pellets have a much lower moisture content than the raw bagasseimproves combustion in the boiler. The bagasse pelletization process basically involve drying, milling,conditioning and cooling. The heat needed for drying before pelletization of the bagasse is assumedto be obtained from flue gas heat recovery and the optimum particle size is recommended to be lessthan 3 mm [15]. More information on the pelletization principle can be found on [9,15]. The idea hereis to pelletize excess bagasse for sale to various customers. Figure 4 illustrates the concept behindthis option.

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Sugar millSugar cane Waste HeatAbsorption

chillerChilled water

Excess bagasse

Surplus electricity

Ele

ctri

c po

wer

Figure 3. Sugar mill waste heat for thermally driven cooling.

2.3.3. Option 3: Utilization of Excess Bagasse for Pellet

Similar to any biomass, bagasse has a bulky nature which makes its handling and transportation a bit difficult. A more compact form of the bagasse in the form of pellets have the advantage of improving its handling and transportation [14]. In addition, bagasse pellets can be stored for utilization during the off-season. The fact that bagasse pellets have a much lower moisture content than the raw bagasse improves combustion in the boiler. The bagasse pelletization process basically involve drying, milling, conditioning and cooling. The heat needed for drying before pelletization of the bagasse is assumed to be obtained from flue gas heat recovery and the optimum particle size is recommended to be less than 3 mm [15]. More information on the pelletization principle can be found on [9,15]. The idea here is to pelletize excess bagasse for sale to various customers. Figure 4 illustrates the concept behind this option.

Figure 4. Utilization of excess bagasse for pellets.

2.3.4. Option 4: Modification of the Cogeneration Unit for Maximum Electric Power Generation

The aim of this option is to maximize electric power generation via dried bagasse and installation of a condensing extraction steam turbine (CEST). The bagasse drying results in reduced moisture content (MC) of the bagasse which in turn leads to improved heating value. This improves the combustion temperature and boiler efficiency. As the final exhaust steam pressure from CEST is below atmospheric, the expansion of live steam down to this pressure will generate more power than if the same amount of live steam with the same conditions were expanded in a back-pressure turbine. In addition, in the case where there is excess bagasse available for use during off-season, the CEST will enable year round generation of electricity for sales to the grid regardless of the absence of sugar/ethanol production. Figure 5 illustrates the concept behind the technological improvement 4.

Figure 4. Utilization of excess bagasse for pellets.

2.3.4. Option 4: Modification of the Cogeneration Unit for Maximum Electric Power Generation

The aim of this option is to maximize electric power generation via dried bagasse and installationof a condensing extraction steam turbine (CEST). The bagasse drying results in reduced moisturecontent (MC) of the bagasse which in turn leads to improved heating value. This improves thecombustion temperature and boiler efficiency. As the final exhaust steam pressure from CEST isbelow atmospheric, the expansion of live steam down to this pressure will generate more powerthan if the same amount of live steam with the same conditions were expanded in a back-pressureturbine. In addition, in the case where there is excess bagasse available for use during off-season, the

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CEST will enable year round generation of electricity for sales to the grid regardless of the absence ofsugar/ethanol production. Figure 5 illustrates the concept behind the technological improvement 4.Sustainability 2016, 8, 1227 7 of 16

Figure 5. Maximum electric power generation using bagasse drying and new turbine setup.

2.4. Wastewater Treatment

As sugar mills generally produce a significant amount of wastewater, the treatment of this in a well-functioning wastewater treatment system is necessary both from environmental point of view as well as the re-use of the treated water for different activities such as irrigation and factory cleaning. The most common wastewater treatment methods include: up flow anaerobic sludge blanket (UASB) reactors, waste stabilization pond system and aerated lagoons. The existing wastewater management system at Carlos Baliño sugar mill is out of function and thus implementing proper wastewater treatment is crucial. Abdulhadi and Larsson [7] have proposed a wastewater management system for Carlos Baliño that consists of two unaerated ponds, each with facultative and maturation ponds. This wastewater treatment measure can also be applied to the other technological improvements as well and needs further investigation. Referring to the technological improvement where thermally driven cooling is considered, there are two main issues related to the contaminants in the wastewater highlighted by Feychting and Vitez [8]. The first one is that the wastewater contains some amount of sugar and this necessitates the continual cleaning of the generator unit of the absorption chilling system. The second issue is that there is a possible contamination of the wastewater by for example metallic residues from the absorption chiller. As there will be a need to re-use the wastewater after the absorption chiller for cleaning and irrigation purposes, treatment of the wastewater becomes inevitable.

Abdulhadi and Larsson [7] had collected and partly measured wastewater parameters of the Carlos Baliño sugar mill. The data can be found in Appendix N in [7]. The average measured values are acidity (pH): 6.4, temperature: 33.7 °C and electric conductivity (EC): 1.05–1.08. The Biological Oxygen Demand (BOD5) data obtained from the sugar mill lies in the range 38.5–2750. The collected data from the sugar mill has shown some variation from the measured values due to improper functioning of the lagoon 2 and also some variation of the wastewater quality during different months. Such fluctuations in the characteristics of the wastewater during different periods the year will negatively affect activities like irrigation. For instance the pH of the wastewater should not be below 6.2 for the proper functioning of anaerobic bacteria and their survival. As per the measurement, the EC indicates the wastewater has average salinity which makes it suitable for irrigation; however, as per the collected data the EC lies between 2.09 and 3.28 showing a big difference from the measured data and that the salinity is unstable. This shows that the wastewater cannot be guaranteed for use for irrigation purposes due to its dehydration effect on young crops given its occasional high salinity. This high salinity of the wastewater calls for the use of additional water supply for lowering the salinity. As Carlos Baliño sugar mill produces organic sugar, there are water quality standards that need to be followed during the process in order to meet both environmental conservation as well as the organic sugar production process requirements. The

Figure 5. Maximum electric power generation using bagasse drying and new turbine setup.

2.4. Wastewater Treatment

As sugar mills generally produce a significant amount of wastewater, the treatment of this in awell-functioning wastewater treatment system is necessary both from environmental point of view aswell as the re-use of the treated water for different activities such as irrigation and factory cleaning.The most common wastewater treatment methods include: up flow anaerobic sludge blanket (UASB)reactors, waste stabilization pond system and aerated lagoons. The existing wastewater managementsystem at Carlos Baliño sugar mill is out of function and thus implementing proper wastewatertreatment is crucial. Abdulhadi and Larsson [7] have proposed a wastewater management systemfor Carlos Baliño that consists of two unaerated ponds, each with facultative and maturation ponds.This wastewater treatment measure can also be applied to the other technological improvements aswell and needs further investigation. Referring to the technological improvement where thermallydriven cooling is considered, there are two main issues related to the contaminants in the wastewaterhighlighted by Feychting and Vitez [8]. The first one is that the wastewater contains some amountof sugar and this necessitates the continual cleaning of the generator unit of the absorption chillingsystem. The second issue is that there is a possible contamination of the wastewater by for examplemetallic residues from the absorption chiller. As there will be a need to re-use the wastewater after theabsorption chiller for cleaning and irrigation purposes, treatment of the wastewater becomes inevitable.

Abdulhadi and Larsson [7] had collected and partly measured wastewater parameters of theCarlos Baliño sugar mill. The data can be found in Appendix N in [7]. The average measured values areacidity (pH): 6.4, temperature: 33.7 ◦C and electric conductivity (EC): 1.05–1.08. The Biological OxygenDemand (BOD5) data obtained from the sugar mill lies in the range 38.5–2750. The collected datafrom the sugar mill has shown some variation from the measured values due to improper functioningof the lagoon 2 and also some variation of the wastewater quality during different months. Suchfluctuations in the characteristics of the wastewater during different periods the year will negativelyaffect activities like irrigation. For instance the pH of the wastewater should not be below 6.2 forthe proper functioning of anaerobic bacteria and their survival. As per the measurement, the ECindicates the wastewater has average salinity which makes it suitable for irrigation; however, as perthe collected data the EC lies between 2.09 and 3.28 showing a big difference from the measured dataand that the salinity is unstable. This shows that the wastewater cannot be guaranteed for use forirrigation purposes due to its dehydration effect on young crops given its occasional high salinity. Thishigh salinity of the wastewater calls for the use of additional water supply for lowering the salinity.As Carlos Baliño sugar mill produces organic sugar, there are water quality standards that need to be

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followed during the process in order to meet both environmental conservation as well as the organicsugar production process requirements. The wastewater generated from organic sugar production hasrelatively lower content of pollutants than that generated from non-organic sugar production. Thus,such wastewater need not be wasted and reutilizing it for some activities should be considered.

2.5. Mass and Energy Balance Relations

Unplanned stoppages are often experienced at the Carlos Baliño sugar mill [7] and there is avariation in the seasonal value of the cane crushed. Thus, it is obvious that operation of the sugar millat nominal parameters is not always the case. The actual operational parameters vary throughout thecrushing season due to unexpected interruptions during the operation of the sugar mill [13]. This leadsto variation of collected data in different crushing season and this is experienced during the in-fieldstudy conducted in different years resulting some difference in the actual operation data collectedfrom the Baliño [7–10]. On the other hand, an actual operation parameter is more interesting for acertain analysis than a nominal value, so the analysis involves using actual operation parameters.

The base case plant data are shown in Table 1.

Table 1. Selected operation parameters for base case plant [10].

Parameter Value Unit Reference

MC 50 % [10]LHVtot,50% MC 7960 kJ/kg [10]

mb,tot 4.25 kg/s [10]mb,ex 0.85 kg/s [10]SGI 2.2 [10]hTEX 2726 kJ/kg [10]hps 293 kJ/kg [10]hSH 3053 kJ/kg [10]

Pel,BC 2.8 a MW [10]mww,ex 10 kg/s [7,8]Tww,ex 96 ◦C [7,8]Timb 70 ◦C [7,8]

a 2 MW is for in-house consumption and 0.8 MW is sold to the grid.

Table 2 summarizes the equations used for calculating some parameters for the differenttechnological options.

Table 2. Equations used for mass and energy balance related calculations.

Parameter Equation Unit

Net bagasse flow.

mb,net =.

mb,tot −.

mb,ex kg/s

Total steam flow.

mst,tot = SGI · .mb,net kg/s

Heat to process.

Qps =.

mst,tot · (hTEX − hFW) kW

Electrical power output.Pel =

.mst,tot · (hSH − hTEX) kW

Lower heating value at a certain bagasse MC a LHVtot,MC = LHVd · (1 − MC)− 2443 · MC kJ/kg

Mass flow of bagasse at a certain bagasse MC.

mb,MC =.

mb,tot ·(1−0.5)(1−MC) kg/s

Fuel energy to the cycle.

Qf =.

mb,net · LHVtot kW

Efficiency of the cycle (BC plant) ηcyc,BC =Pel+

.Qps

.Qf

· 100 %

Efficiency of the cycle for a specific technological option ηcyc,opt =Pel+

.Qps+

( .Esav,opttopt ·24

).

Qf· 100 %

a For Option 4, the dry basis LHV (Lower Heating Value) is estimated based on HHV (Higher Heating Value)provided in [10].

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The energy saving for Options 1, 3 and 4 is calculated based on the equations provided in Table 3;for Option 2, data are taken from Feychting and Vitez [8].

Table 3. Equations used for calculating energy saving for options 1, 3 and 4.

Parameter Equation Unit

Energy savings for Option1 Esav,opt1 =.

mww,imb · cp,w · (Tww,ex − Timb) · topt1 · 24 MWh/year

Energy savings for Option 2 Esav,opt2 = 140 [8] MWh/year

Energy savings for Option 3 Esav,opt3 =.

mb,ex,prac · LHVtot,50%MC · topt3 · 24 MWh/year

Energy savings for Option 4 a Esav,opt4 =( .mb,tot −

.mb,MC

)· LHVtot,MC · topt4 · 24 MWh/ year

a The MC (moisture content) used is 35%.

The equations used for calculating key the economic parameters used in the analysis of this studyare provided in Table 4. The electricity price is varied from 0.25 to 0.34 USD/kWh to see its influenceon the financial gain and DPBP (discounted payback period).

Table 4. Summary of equations used for calculations related to economic parameters.

Parameter Equation Unit

Cost of capital a Ccapital =(

Revopt − Cm&o)· 1−(1+r)−n

r − Itot [8,9] USD/year

Yearly net profit b Pro f ityearly = Revopt − Cm&o USD/year

Maintenance and operationcost for option 1 c

-Maintenance cost c Cm,opt1 = 0.1 · Itot,opt1 USD/year

-Energy price d Eprice,opt1 =∼Vimb,w·H·ρw·g·λel·topt2·24

3.6·106·ηp·ηmUSD/year

a By setting the cost of capital to zero and solving for n, the discounted payback period (DPBP) is estimated;b This is the same as the financial gain for Options 1–4; c The labour cost is negligible. The maintenance costis assumed to be 10% per year of the initial investment cost [16]; d The volume flowrate is estimated about22.5 m3/h based on the additional 6 kg/s imbibition water [7]. The water is assumed to be under a 3 mhead [17] and the density taken at 96 ◦C. The pump and electric motor efficiencies are assumed to be 90% each.The acceleration due to gravity is 9.81 m/s2.

3. Results

In this section, the comparison results for the four technological options are presented.

3.1. Summary of Key Results from the Four Technological Options

3.1.1. Option 1

Table 5 provides some of the main results for Option 1. The excess wastewater flow was calculatedto be 10 kg/s with a temperature of 96 ◦C. In the study by Abdulhadi and Larsson [7], the wastewaterwas considered to be partly utilized for imbibition and partially available for a wastewater managementsystem which will be generating treated water for irrigation. The crushing season was considered to be140 days per year. The amount of the optimized imbibition water was estimated as 11 kg/s whereas thebase case imbibition water flow was calculated to be 5.6 kg/s. The amount of wastewater remainingfor wastewater management system was estimated to be 4 kg/s. The total sugar production after theoptimized imbibition water flow is used is estimated to be 2.5 kg/s which is an 8.7% increase from thebase case sugar production. The model result of the study indicated that there will be a net financialgain of USD 1.4 million due to the optimized imbibition flow and thus increased sugar production.

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Table 5. Results for Option 1.

Parameter Value Unit Reference

Part of excess wastewater reused for imbibition, mww,imb 6 kg/s [7]Optimized imbibition water flow, moptim,imb 11 kg/s [7]

Crushing season, topt1 140 Days [7]Revenue from sugar produced, Revs

a 1.4 106 USD/year [7]Investment cost, Iopt1

b 3500 USD [18]Maintenance and operating cost, Cm&o

c 570 USD/yeara Calculated value and refers to the net revenue from the increased sugar production; b cost of the pump neededfor the additional imbibition water; c calculated value using equations in Table 4.

3.1.2. Option 2

The main results for Option 2 are summarized in Table 6. The excess water flow was calculatedto be 10 kg/s with a temperature of 96 ◦C. The maximum cooling demand is estimated to be 288 kW.Energy saving per crushing season is 140,000 kWh, which corresponds to the current electricityconsumption of the vapor compression units (equivalent of 40,000 USD/season). The potentialrevenue from excess electricity production of the base case plant is USD 800,640 per year based onan electricity price of 0.3 USD/kWh and 0.8 MW peak export capacity. In the study for this option,the crushing season is considered to be 139 days per year. Considering the 140,000 kWh, financialsavings of USD 40,000 per year with a payback of 3–6 years (depending on the type of equipmentselected) were estimated.

Table 6. Results for Option 2 as taken from [8].

Parameter Value Unit

Total bagasse flow, mb,tot 8.8 a kg/sEnergy saving per crushing season, Esav,opt1 140,000 b kWh

Crushing season, topt1 139 DaysTotal investment cost, Itot 152,326 USD/year

Maintenance and operating cost, Cm&o 1171 USD/yearRevenue c, Revabs 41,490 USD/year

a When estimating the excess bagasse flow, the ratio of the excess bagasse and total bagasse flow presented inTable 1 will be considered; b Corresponds to the current electricity consumption of the vapor compression units;c Due to saved electricity after installing thermally driven absorption chiller.

3.1.3. Option 3

Some of the main results for Option 3 as taken from [9] are summarized in Table 7. The studymodel results for the third technological option showed that the seasonal practical amount of excessbagasse was 11,500 t/season, whereas the seasonal nominal amount of excess bagasse was estimated tobe 37,000 t/season. Considering the practical amount of excess bagasse, the study result indicated that7300 ton of pellets/year can be produced. The investment cost was calculated to be USD 3.4 millionand the crushing season was considered to be 125 days per year.

Table 7. Results for Option 3 as taken from [9].

Parameter Value Unit

Practical amount of excess bagasse flow, mb,ex,prac 11.5 103 tonnes/yearNominal amount of excess bagasse flow, mb,ex,nom 37 103 tonnes/year

Yearly pellet production capacity, mpellet 7.3 a 103 tonne/yearCrushing season, topt3 125 Days

Total investment cost, Itot 1307 103 USD/yearMaintenance and operating cost, Cm&o 363 103 USD/year

Revenue, Revpellet 1068 a 103 USD/yeara Considering the practical amount of excess bagasse.

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3.1.4. Option 4

This selected scenario considers the bagasse drying and installation of a condensing extractionsteam turbine. Key results for Option 4 are provided in Table 8. The results from this scenario showedthat the maximum surplus electrical energy reaches close to 13.4 GWh and the investment cost forinstalling a new dryer and turbine is estimated to be USD 6.3 million with a payback time of 1.7 years.The yearly profit is estimated to be USD 3.7 million. The crushing season is considered to be 140 daysper year.

Table 8. Results for Option 4 as taken from [10].

Parameter Value Unit

Excess power capacity, Pel,ex 3.9 MWCrushing season, topt4 140 Days

Total investment cost, Itot 5045 103 USD/yearMaintenance and operating cost, Cm&o 195 103 USD/year

Revenue, Revel,ex 3931 103 USD/year

3.1.5. Comparison of the Different Technological Options

In all four technological improvement studies, the electricity sales price was considered to be0.3 $/kWh, and for detailed sensitivity analysis of the electricity price along with other parameters thereader is referred to [8–10]. As per the calculation results from this study, for Option 1, the variationof the electricity price from 0.25 to 0.34 USD/kWh does not produce a significant variation in thecalculated net financial gain and DPBP. For Option 2, the net financial gain and the DPBP vary from33,404 to 45,851 USD/year and 8 to 5 years, respectively. For Option 3, the net financial gain and theDPBP vary from 526,821 to 847,195 USD/year and 3 to 2 years, respectively. For Option 4, the netfinancial gain and the DPBP vary from 3 million to 4.3 million USD/year and 2 to 1.4 years, respectively.For all the options, the discount rate was considered to be 15% as Cuba is a developing country andcompared to developed countries there is lower risk in the economy [7–9].

The results of the calculated parameters based on data provided in Table 1 and using equationsin Table 2 are presented in Table 9. The table also shows cycle efficiency as considered for the fouroptions. As can be seen in the table, the efficiency of the cycle is more sensitive to the technologicalimprovements Option 3 and 4 as compared to the other two options.

Table 9. Calculated parameters and cycle efficiency for the four options.

Parameter Unit Option 1 Option 2 Option 3 Option 4

Mass flow of net bagasse kg/s 7.0 a 7.0 a 3.7 3.3

Total steam flow, BC is kg/s 15.5 b 15.5 b 8.1 b 11.4

Fuel input MW 56.0 56.0 29.3 27.1 c

Electrical power output(2 MW in-house and 0.8 MW is sold) MW 2.8 2.8 1.6 2.8

Process heat demand MW 37.7 37.7 19.7 18.2

Efficiency of cycle (BC), LHV basis % 72.2 72.2 72.8 77.6

Efficiency cycle (Options), LHV basis % 73.4 72.8 100.4 106.1a Estimated by taking ration of 0.85 to 4.25 from Carlos Baliño data and then recalculating from the 8.8 kg/s ofOption 1 and 2; b Steam: bagasse = 2.2 (SGI) is considered for options 1–3 and the total amount of bagasse isconsidered with 35% MC for Option 4; c LHV (@35% MC) 9682.6 kJ/kg and Qf (@35% MC) = 31.6 MW.

Based on the input data collected from the four studies [7–10] and the calculated parameters inTable 9, two comparison charts are generated as shown in Figures 6 and 7. Figure 6 illustrates theenergy savings and cycle efficiency increments of all four options.

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Figure 6. Energy saving and cycle efficiency increment (relative to base case) for the four options.

Figure 7 illustrates the financial saving per year and the DPBP for the four options. The cycle efficiency for Options 3 and 4 are relatively higher than for the other options as surplus power generation (Option 4) and pellets (Option 3) are produced.

Figure 7. Financial savings and DPBP (discount payback period) for the four options.

For options 3 and 4, the contribution of the technological improvement to minimize emission is obvious in that the use of bagasse for power generation is a CO2-neutral alternative, whereas for Options 1 and 2, in addition to heat recovery, the reuse of part of the wastewater reduces BOD and Chemical Oxygen Demand (COD) concentrations in the river where it is disposed. In addition, for Option 2, the implementation of heat driven absorption cooling system instead of the existing vapour compression systems that are driven by fossil fuel generated electricity, the CO2 emission is calculated by considering the amount of electric power (140,000 kWh) consumed by the existing vapour compression systems. Similarly, for Options 3 and 4, the CO2 emission savings are calculated based on the amount of energy saved per crushing season. The amount of CO2 from fuel oil-based power generation considered in the calculation of the CO2 emission saved equals 675 g CO2/kWh [19]. Figure 8 illustrates the CO2 emissions saved in tonnes/year and the cost of CO2 reduction per tonne of CO2 for Options 2–4. The CO2 emissions associated with Option 1 are unaffected by the technological improvement made; however, the sugar yield increases. Option 3 represents the most cost-effective CO2 reduction technology, although its level is much higher than benchmarks like the EU Emissions Trading Scheme [20].

As can be seen from Figures 6 and 8, the amount of CO2 emission saved corresponds to the amount of energy saved for the respective options. The CO2 emissions saved for Option 2 is quite

Figure 6. Energy saving and cycle efficiency increment (relative to base case) for the four options.

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Figure 6. Energy saving and cycle efficiency increment (relative to base case) for the four options.

Figure 7 illustrates the financial saving per year and the DPBP for the four options. The cycle efficiency for Options 3 and 4 are relatively higher than for the other options as surplus power generation (Option 4) and pellets (Option 3) are produced.

Figure 7. Financial savings and DPBP (discount payback period) for the four options.

For options 3 and 4, the contribution of the technological improvement to minimize emission is obvious in that the use of bagasse for power generation is a CO2-neutral alternative, whereas for Options 1 and 2, in addition to heat recovery, the reuse of part of the wastewater reduces BOD and Chemical Oxygen Demand (COD) concentrations in the river where it is disposed. In addition, for Option 2, the implementation of heat driven absorption cooling system instead of the existing vapour compression systems that are driven by fossil fuel generated electricity, the CO2 emission is calculated by considering the amount of electric power (140,000 kWh) consumed by the existing vapour compression systems. Similarly, for Options 3 and 4, the CO2 emission savings are calculated based on the amount of energy saved per crushing season. The amount of CO2 from fuel oil-based power generation considered in the calculation of the CO2 emission saved equals 675 g CO2/kWh [19]. Figure 8 illustrates the CO2 emissions saved in tonnes/year and the cost of CO2 reduction per tonne of CO2 for Options 2–4. The CO2 emissions associated with Option 1 are unaffected by the technological improvement made; however, the sugar yield increases. Option 3 represents the most cost-effective CO2 reduction technology, although its level is much higher than benchmarks like the EU Emissions Trading Scheme [20].

As can be seen from Figures 6 and 8, the amount of CO2 emission saved corresponds to the amount of energy saved for the respective options. The CO2 emissions saved for Option 2 is quite

Figure 7. Financial savings and DPBP (discount payback period) for the four options.

Figure 7 illustrates the financial saving per year and the DPBP for the four options. The cycleefficiency for Options 3 and 4 are relatively higher than for the other options as surplus powergeneration (Option 4) and pellets (Option 3) are produced.

For options 3 and 4, the contribution of the technological improvement to minimize emissionis obvious in that the use of bagasse for power generation is a CO2-neutral alternative, whereasfor Options 1 and 2, in addition to heat recovery, the reuse of part of the wastewater reduces BODand Chemical Oxygen Demand (COD) concentrations in the river where it is disposed. In addition,for Option 2, the implementation of heat driven absorption cooling system instead of the existingvapour compression systems that are driven by fossil fuel generated electricity, the CO2 emissionis calculated by considering the amount of electric power (140,000 kWh) consumed by the existingvapour compression systems. Similarly, for Options 3 and 4, the CO2 emission savings are calculatedbased on the amount of energy saved per crushing season. The amount of CO2 from fuel oil-basedpower generation considered in the calculation of the CO2 emission saved equals 675 g CO2/kWh [19].Figure 8 illustrates the CO2 emissions saved in tonnes/year and the cost of CO2 reduction per tonne ofCO2 for Options 2–4. The CO2 emissions associated with Option 1 are unaffected by the technologicalimprovement made; however, the sugar yield increases. Option 3 represents the most cost-effectiveCO2 reduction technology, although its level is much higher than benchmarks like the EU EmissionsTrading Scheme [20].

As can be seen from Figures 6 and 8, the amount of CO2 emission saved corresponds to the amountof energy saved for the respective options. The CO2 emissions saved for Option 2 is quite small ascompared to the emissions saved from the other options and are estimated to be 94.5 tonnes/year.

Sustainability 2016, 8, 1227 13 of 16

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small as compared to the emissions saved from the other options and are estimated to be 94.5 tonnes/year.

Figure 8. Amount of CO2 emission saved and cost of CO2 reduction per tonne CO2 for Options 2–4.

The choice of which technological option should be prioritized depends on its relevance to the current Carlos Baliño sugar mill situation. The advantage with implementing Option 3 and Option 4 together can be that taking into account the seasonality of sugar production, the pellets can be saved for off-season and can either be sold or used for power generation by the sugar mill during off-season. In addition, during unexpected interruptions, the pellets can be used as a back-up fuel. Considering Option 2, though the cycle efficiency is not affected much by the re-use of the waste heat in the wastewater for absorption cooling technology. However, the implementation of Option 2 is a viable technology and worth investment if the priority of Carlos Baliño sugar mill is to have a cooling supply which is based on a renewable energy resource. On the other hand, Option 2 is reliable only for the crushing season and so does not guarantee a cooling supply during off-season.

4. Conclusions

The purpose of this study was to compare the four technological options that were proposed for the Carlos Baliño sugar mill and suggest which option is suitable to implement. For this, different factors are considered as comparison parameters, and the implementation of technological improvements 3 and 4 have more sensitivity to the improvement of the cycle efficiency. The cycle efficiencies calculated for Option 3 and Option 4 are 37.9% and 36.8%, respectively. This alone, however, does not necessarily lead to the conclusion that these options need to be prioritized compared to Option 1 and Option 2. Option 2 can be attractive, if an improved and environmentally friendly air conditioning system for the factory is prioritized regardless of the longer DPBP compared to the other options. The prioritizing of the four options was examined from other points of views as well: financial gains, DPBP and energy saving. The CO2 emissions saved due to Option 4 are the highest (21,538 tonnes/year) and for Option 2 it is the lowest (94.5 tonnes/year). The cost of CO2 emissions saved for Options 2 (highest) and 3 (lowest) is estimated to be 423 and 41 USD/tonne of CO2 emissions saved, respectively.

Acknowledgments: Many thanks for the financial support of the Swedish International Development Cooperation Agency (SIDA) that funds this project. Thanks to all from Carlos Baliño sugar mill staff for providing data and necessary information during the field study. Our thanks to the scholarship providers from SIDA’s Minor Field Studies (MFS) in developing countries programme and Din Els Miljöfond, which funded the field trips to Cuba. Publishing costs were assumed by KTH, Division of Heat and Power Technology.

Author Contributions: E. Birru has substantially contributed to the writing of the manuscript, the analysis regarding the comparison of the technological options considered in this study and to the interpretation of results. C. Erlich and A. Martin have contributed to the design of the study and to the preparation of the manuscript. I. Herrera provided data from Carlos Baliño sugar mill. The remaining authors have contributed

Figure 8. Amount of CO2 emission saved and cost of CO2 reduction per tonne CO2 for Options 2–4.

The choice of which technological option should be prioritized depends on its relevance to thecurrent Carlos Baliño sugar mill situation. The advantage with implementing Option 3 and Option4 together can be that taking into account the seasonality of sugar production, the pellets can besaved for off-season and can either be sold or used for power generation by the sugar mill duringoff-season. In addition, during unexpected interruptions, the pellets can be used as a back-up fuel.Considering Option 2, though the cycle efficiency is not affected much by the re-use of the waste heatin the wastewater for absorption cooling technology. However, the implementation of Option 2 is aviable technology and worth investment if the priority of Carlos Baliño sugar mill is to have a coolingsupply which is based on a renewable energy resource. On the other hand, Option 2 is reliable only forthe crushing season and so does not guarantee a cooling supply during off-season.

4. Conclusions

The purpose of this study was to compare the four technological options that were proposed for theCarlos Baliño sugar mill and suggest which option is suitable to implement. For this, different factorsare considered as comparison parameters, and the implementation of technological improvements3 and 4 have more sensitivity to the improvement of the cycle efficiency. The cycle efficiencies calculatedfor Option 3 and Option 4 are 37.9% and 36.8%, respectively. This alone, however, does not necessarilylead to the conclusion that these options need to be prioritized compared to Option 1 and Option 2.Option 2 can be attractive, if an improved and environmentally friendly air conditioning system forthe factory is prioritized regardless of the longer DPBP compared to the other options. The prioritizingof the four options was examined from other points of views as well: financial gains, DPBP and energysaving. The CO2 emissions saved due to Option 4 are the highest (21,538 tonnes/year) and for Option 2it is the lowest (94.5 tonnes/year). The cost of CO2 emissions saved for Options 2 (highest) and 3(lowest) is estimated to be 423 and 41 USD/tonne of CO2 emissions saved, respectively.

Acknowledgments: Many thanks for the financial support of the Swedish International Development CooperationAgency (SIDA) that funds this project. Thanks to all from Carlos Baliño sugar mill staff for providing data andnecessary information during the field study. Our thanks to the scholarship providers from SIDA’s Minor FieldStudies (MFS) in developing countries programme and Din Els Miljöfond, which funded the field trips to Cuba.Publishing costs were assumed by KTH, Division of Heat and Power Technology.

Author Contributions: E. Birru has substantially contributed to the writing of the manuscript, the analysisregarding the comparison of the technological options considered in this study and to the interpretation of results.C. Erlich and A. Martin have contributed to the design of the study and to the preparation of the manuscript.I. Herrera provided data from Carlos Baliño sugar mill. The remaining authors have contributed via BSc thesiswork: B. E. Abdulhadi and A. Larsson (Option 1 study); S. Feychting and F Vitez (Option 2 study); E. Onoszkoand M. Hallerbo (Option 3 study); and L. Weilenmann and L. Puskoriute (Option 4 study).

Conflicts of Interest: The authors declare no conflict of interest. The funding sponsors had no role in the designof the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, and in thedecision to publish the results.

Sustainability 2016, 8, 1227 14 of 16

Nomenclature

Character Parameter Unitm Mass flow kg/s.

Q Heat flow kW or MWP Power kW or MWη Efficiency %h Enthalpy kJ/kgE Energy GWhcp Specific heat capacity kJ/kg·◦CC Cost USD/yearn Period yearsr Discount rate %I Investment USDG Financial gain USD/yearRev Revenue USD/yeart Crushing season daysV Volume flow m3/hλ Electricity price USD/kWhρ Density kg/m3

g Standard gravity m/s2

H head m

Superscripts

b BagasseBC Base Case plantex Excessel Electricityf Fuelnet Net mass flowm Maintenanceopt Optionps Processst SteamSH SuperheatedTot TotalTex Turbine exhaustFW Feed waterd Dry basisOptim OptimumCyc Cyclesav Savingww Wastewaterimb imbibitionnom nominalw waterprac practicalm&o Operation and maintenances sugarm maintenanceabs absorption chiller

Sustainability 2016, 8, 1227 15 of 16

Abbreviations

CEST Condensing extraction steam turbineMC Moisture contentBOD Biological Oxygen DemandDPBP Discounted Payback PeriodTCD Tonne of cane per dayCOD Chemical Oxygen DemandSGI Steam Generation IndexLHV Lower Heating ValueHHV Higher Heating Value

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© 2016 by the authors; licensee MDPI, Basel, Switzerland. This article is an open accessarticle distributed under the terms and conditions of the Creative Commons Attribution(CC-BY) license (http://creativecommons.org/licenses/by/4.0/).